春草がカンキツ樹の窒素の肥効に及ぼす影響

Abstract

This experiment was carried out to clarify the effect of spring weeds on the uptake of fertilizer nitrogen which had been applied on March 5. The citrus orchard was located on a site with a gentle slope and where 8-year-old satsuma trees were grown. The soil had originated from granite.In the bare ground plot, all of the weeds were wiped out with herbicide in the early spring. In the weed plot, most of the weed roots were found in the surface soil 0 to 5cm deep, while most of the satsuma roots were below 5cm. The soil temperature was consistently higher in the bare ground plot than in the weed plot, and was higher at the soil surface than in the subsoil. At a depth of 10cm, it reached 12°C in the middle of March to early April. In the bare ground plot, this temperature was reached 20 days earlier.The spring weeds, when cut and weighed on May 12, amounted to 3.23 tons per 10a. Total nitrogen content of the weeds was 10.5kg per 10a, and 38.6% of 15N applied in the early spring was absorbed by the weeds. After the cut weeds were mulched, about 75% of 15N had disappeared by September. This nitrogen was probably transferred into the soil and was partly reabsorbed by the satsuma trees. Thus, in the weed plot, 15N content in the leaves and fruit of the tree increased after September, while in the bare ground plot, the 15N content in the flowers and leaves was higher in early summer.In the bare ground plot, soil carbon and nitrogen contents were very low on May 12. 15N content in the subsoil of the bare ground plot was high in inorganic matter on May 12, but decreased markedly by July 21. In contrast, the 15N content in the surface to subsoil of the weed plot did not decrease as drastically during this period.The total amount of 15N absorbed by the trees over 13 months was very small compared with the 15N remaining in the soil. Nearly 90% of the 15N in the soil was found in the surface soil. The percentages of 15N remaining in the soil in the bare ground and weed plots were 29.7% and 63.6%, respectively.In conclusion, the pathway of nitrogen applied in early spring to citrus orchards differs considerably from spring to early summer depending on whether or not the ground is weed coverd.